The Reagan administration's surprise decision to assemble neutron warheads has complicated West German Chancellor Helmut Schmidt's efforts to portray President Reagan as seriously committed to nuclear arms reductions and given new ammunition to European pacifist attempts to portray the United States, rather than the Soviet Union, as the more aggressive superpower.

The wisdom of the decision is being questioned for political rather than military reasons.

Despite claims by U.S. Defense Secretary Caspar W. Weinberger that no time would have been a trouble-free time to make the announcement, it follows close on the heels of recent strenuous efforts by Schmidt to dispel European anxieties.

These center around a Western security concern viewed as far more important than the neutron controversy -- the planned deployment in Europe of new medium-range nuclear missiles.

What gains peace groups may have made against the NATO missiles decision cannot be fully assessed yet, according to officials. Antimissile campaigns, now in a summer lull, are expected to gather force again in the autumn.

But Washington's announcement about the controversial neutron weapon is seen as compounding the already intense and politically preoccupying discussion of Western defense strategies in West-Central Europe. U.S. Secretary of State Alexander M. Haig Jr. reportedly warned Weinberger and Reagan of this possibility, and Bonn officials regard Haig as the Cabinet member most sensitive to the political implications in Europe of U.S. actions.

Clearly trying to limit the extent of political fallout here from the U.S. decision, the West German government has sought to distance itself from the American action. The Netherlands, Belgium and Italy have also been fending off antimissile campaigns with varying success.

They have stressed that production of the neutron warhead was a matter for the United States alone to decide, that the weapon becomes an alliance issue only when deployment outside America is proposed.

Having been handed a fait accompli, West Germany has also moved quickly to hold out the hope that the neutron weapon can be included in some future East-West disarmament negotiations. Its Foreign Ministry is studying how the warhead might be bargained against Soviet tanks in Europe and sizable stockpiles of Soviet chemical weapons.

Based on Western alliance military doctrines alone, such as flexible response, West German defense experts see reason for the addition of enhanced radiation weapons to the U.S. arsenal. Although this country has an instinctive aversion to battlefield nuclear weapons, since they would most likely be deployed in West Germany in the event of a European war, the powerful deterrent value of the neutron warhead also seems little doubted by Bonn officials.

But an irony of NATO's shift from weaponry of massive retaliation to flexible response is more acute Western European anxiety over the view that the United States can confine a nuclear war to Europe without suffering itself.

Reflecting such arguments, Egon Bahr, a senior member and strategist in Schmidt's Social Democratic Party, stated yesterday that the "miniaturization" of weapons would make the escalation from conventional to nuclear war easier.

Defense experts in the chancellery admit to having questions themselves on this score, particularly about the concept -- new to some of them -- of an "integrated battlefield." This notion, which says that a mix of conventional, nuclear and chemical fighting tactics should be readied, causes misgivings here about the conditions under which escalation into nonconventional weapons is practiced.

"The arguments about escalation and control of battlefield weapons become very hypothetical," said a Bonn government defense expert. "You then end up having too much public discussion. This does not add to deterrence. It adds to doubt."

The United States can now be more readily accused by European peace campaigners of accelerating the arms race. It is also being pictured by the West German Social Democratic Party, the Dutch Foreign Ministry and others as acting high-handedly in reaching the production decision without NATO consultation.

"Mr. Weinberger has argued for the manufacture of the weapon partly on the grounds that the U.S. cannot have its defense policy dictated by the Europeans," asserted the London Financial Times today. "This argument does not say much for his interest in the spirit of the NATO alliance."